1. Field of the Invention
The present invention relates to nonreciprocal circuit elements, and, more particularly, to a nonreciprocal circuit element such as an isolator or a circulator used in a microwave band.
2. Description of the Related Art
A nonreciprocal circuit element such as an isolator or a circulator has a characteristic of transmitting a signal in only a predetermined direction and transmitting no signal in the opposite direction, and is used in, for example, a transmission circuit of a mobile communication device such as a car phone or a mobile phone.
WO Publication No. 2009/028112 discloses, as this kind of nonreciprocal circuit element, a two-port isolator in which a first center electrode and a second center electrode intersect and are insulated from each other on a ferrite surface and an LC series resonant circuit including a capacitor and an inductor is connected in parallel to the first center electrode and is connected in series to a terminating resistor. When high-frequency power is input into this two-port isolator from an inverse direction, the impedance characteristics of the terminating resistor and the LC series resonant circuit achieve matching in a wide frequency band. As a result, an isolation characteristic is improved. On the other hand, when high-frequency power is input into this two-port isolator from a forward direction, the high-frequency power hardly flows through the first center electrode and the terminating resistor. Accordingly, the degradation in an insertion loss due to the addition of the LC series resonant circuit can be ignored.
In the two-port isolator, the inductor included in the LC series resonant circuit needs to have an inductance value in the range of approximately 60 nH to approximately 80 nH. It is assumed that a chip coil with a length of approximately 0.6 mm, a width of approximately 0.3 mm, and a height of approximately 0.3 mm is used as an inductor having the above-described inductance value. In this case, since the self-resonance frequency of the chip coil is approximately 1 GHz, the chip coil cannot be used in a nonreciprocal circuit element that operates at a frequency equal to or larger than approximately 1 GHz. This problem can be solved by connecting a plurality of chip coils having a small inductance value in series or using a large-sized chip coil whose self-resonance frequency is high.
However, this leads to increases in a product size and a cost. In addition, since the allowable current of a chip coil is reduced with the increases in an inductance value, the conductor of the chip coil may be broken by high-frequency power reflected from an antenna. This leads to unreliability.
On the other hand, the capacitor included in the LC series resonant circuit needs to have a small capacitance value in the range of approximately 0.1 pF to approximately 0.4 pF. However, in a capacitor having a small capacitance value, an effective capacitance value is significantly changed because of the variation in a stray capacitance, which cannot be avoided, and an isolation characteristic varies greatly. It is therefore difficult to stably mass-produce nonreciprocal circuit elements having a desired characteristic.